We have been investigating the mechanism/s by which membrane proteins are turned over in the endoplasmic reticulum. Attention has focused on CYP2E1, a P-450 enzyme that is induced by alcohol and is thought to enhance lipid peroxidation and thereby liver damage during alcohol consumption. Antigen processing and degradation of this protein and other related forms are mediated by a common mechanism. It is therefore of current interest to determine whether the balance between the production of antigenic peptides and proteolytic processing in the endoplasmic reticulum are mutually dependent events.